I would recommend reading the first iteration, Soul Land. I read the rest in chronilogical order, you don't have to but it can help with understanding certain events and systems. I rank them Soul Land IV, Soul Land II, Soul Land I, Soul Land III. I don't dislike Soul Land III just a bit slow and not as updated compared to the rest. There's an additional spin-off, Soul Land - Legend of Tangs' Hero, I haven't read it yet, but you can read it too if you want to.
Read doulo daly first then soul land
1) Douluo Dalu
2) Douluo Dalu 2 - combat_continent
3) Soul Land - Legend of The Gods' Realm (Side Story)
4) Douluo Dalu 3 - The Legend of the Dragon King
5) Soul Land - the ultimate combat
there's one more missing in the list, this is the continuation for SL 2.5 Legend of the God's realm
http://www.mangago.zone/read-manga/soul_land_legend_of_tangs_hero/
Does anyone know the order to read the series?